Fullerenes are closed-cage molecules composed entirely of sp2-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) were first identified as closed spheroidal cages produced by condensation from vaporized carbon. Fullerene tubes are produced in carbon deposits on the cathode in carbon arc methods of producing spheroidal fullerenes from vaporized carbon. Ebbesen et al., Nature, 1992, 358:220 and Ebbesen et al., Annual Review of Materials Science, 1994, 24:235-264. Such tubes are referred to herein as carbon nanotubes. Many of the carbon nanotubes made by these processes were multi-wall nanotubes (MWNTs), i.e., the carbon nanotubes resembled concentric cylinders having multiple walls or shells arranged in a manner which can be considered analogous to Russian “nesting dolls.” Carbon nanotubes having up to seven walls have been described in the prior art (Ebbesen et al., Annual Review of Materials Science, 1994, 24:235-264; Iijima et al., Nature, 1991, 354:56-58).
Single-wall carbon nanotubes (SWNTs) were discovered in 1993 in soot produced in an arc discharge in the presence of transition metal catalysts. Such SWNTs, comprised of a single tube of carbon atoms, are the smallest of the carbon nanotubes. SWNTs can typically have lengths of up to several micrometers (millimeter-long nanotubes have been observed) and diameters of approximately 0.5 nm-10.0 Onm (Saito et al., Physical Properties of Carbon Nanotubes, 1998, London: Imperial College ////Press; Sun et al., Nature, 2000, 403:384), although most have diameters of less than 2 nm (Saito et al.). Diameters as small as 0.4 nm have been reported, but these were formed inside either MWNTs (Qin et al., Chem. Phys. Lett., 2001, 349:389-393) or zeolites (Wang et al., Nature, 2000, 408:50-51). SWNTs, and carbon nanotubes of all types have since been produced by other techniques which include chemical vapor deposition techniques (Hafner et al, Chem. Phys. Lett., 1998, 296:195-202; Cheng et al., Chem. Phys. Lett., 1998, 289:602-610; Nikolaev et al., Chem. Phys. Lett., 1999, 313:91-97), laser ablation techniques (Thess et al., Science, 1996, 273:483487), and flame synthesis (Vander Wal et al., J. Phys. Chem. B., 2001, 105(42): 10249-10256).
Since their discovery, there has been a great deal of interest in the functionalization (sometimes referred to as derivatization) of carbon nanotubes and, more particularly, in single-wall carbon nanotubes, to facilitate their manipulation, to enhance the solubility of such nanotubes, and to make the nanotubes more amenable to blend and composite formation. This is because single-wall carbon nanotubes are one of the more striking discoveries in the chemistry and materials genre in recent years. Nanotubes posses tremendous strength, an extreme aspect ratio, and are excellent thermal and electrical conductors. A plethora of potential applications for nanotubes have been hypothesized, and some progress is being made towards commercial applications. Accordingly, chemical modification of single-wall carbon nanotubes, as well as multi-wall carbon nanotubes, will be necessary for some applications. For instance, such applications may require grafting of moieties to the nanotubes: to allow assembly of modified nanotubes, such as single-wall carbon nanotubes, onto surfaces for electronics applications; to allow reaction with host matrices in polymer blends and composites; and to allow the presence of a variety of functional groups bound to the nanotubes, such as single-wall carbon nanotubes, for sensing applications. And once blended, some applications may benefit from the thermal removal of these chemical moieties, as described in PCT publication WO 02/060812 by Tour et al., filed Jan. 29, 2002 and incorporated by reference herein.
While there have been many reports and review articles on the production and physical properties of carbon nanotubes, reports on chemical manipulation of nanotubes have been slow to emerge. There have been reports of functionalizing nanotube ends with carboxylic groups (Rao, et al., Chem. Commun., 1996,1525-1526; Wong, et al., Nature, 1998, 394:52-55), and then further manipulation to tether them to gold particles via thiol linkages (Liu, et al., Science, 1998, 280:1253-1256). Haddon and co-workers (Chen, et al., Science, 1998, 282:95-98) have reported solvating single-wall carbon nanotubes by adding octadecylamine groups on the ends of the tubes and then adding dichlorocarbenes to the nanotube sidewall, albeit in relatively low quantities (˜2%).
Success at covalent sidewall derivatization of single-wall carbon nanotubes has been limited in scope, and the reactivity of the sidewalls has been compared to the reactivity of the basal plane of graphite. Aihara, J. Phys. Chem. 1994, 98:9773-9776. A viable route to direct sidewall functionalization of single-wall carbon nanotubes has been fluorination at elevated temperatures, which process was disclosed in a co-pending application commonly assigned to the assignee of the present Application, U.S. patent application Ser. No. 09/810,390, “Chemical Derivatization Of Single-Wall Carbon Nanotubes To Facilitate Solvation Thereof; And Use Of Derivatized Nanotubes To Form Catalyst-Containing Seed Materials For Use In Making Carbon Fibers,” to Margrave et al., filed Mar. 16, 2001. These functionalized nanotubes may either be de-fluorinated by treatment with hydrazine or allowed to react with strong nucleophiles, such as alkyllithium reagents. Although fluorinated nanotubes may well provide access to a variety of functionalized materials, the two-step protocol and functional group intolerance to organolithium reagents may render such processes incompatible with certain, ultimate uses of the carbon nanotubes. Other attempts at sidewall modification have been hampered by the presence of significant graphitic or amorphous carbon contaminants. Chen, Y. et al., J. Mater Res. 1998, 13:2423-2431. For some reviews on sidewall functionalization, see Bahr et al., J. Mater. Chem., 2002, 12:1952; Banerjee et al., Chem. Eur. J., 2003, 9:1898; and Holzinger et al., Angew. Chem. Int. Ed., 2001, 40(21): 4002-4005. Within the literature concerning sidewall-functionalization of SWNTs, however, there is a wide discrepancy of solubility values between reports. This is due to explicable variations in filtration methods.
A more direct approach to high degrees of functionalization of nanotubes (i.e., a one step approach and one that is compatible with certain, ultimate uses of the nanotubes) has been developed using diazonium salts and was disclosed in a co-pending application commonly assigned to the assignee of the present Application. See PCT publication WO 02/060812 by Tour et al., filed Jan. 29, 2002 and incorporated herein by reference. Using pre-synthesized diazonium salts, or generating the diazonium species in situ, reaction with such species has been shown to produce derivatized SWNTs having approximately 1 out of every 20 to 30 carbons in a nanotube bearing a functional moiety. Nevertheless, because of the poor solubility of SWNTs in solvent media, such processes require extraordinary amounts of solvent for the dissolution and/or dispersion of the SWNTs (˜2 L/g coupled with sonication in most cases). See Bahr et al., Chem. Commun., 2000, 193-194, incorporated herein by reference. This problem of an inordinate amount of solvent makes covalent functionalization on the industrial scale economically infeasible.
Solvent-free reactions have been used for a variety of systems. Such solvent-free conditions are generally superior in systems for which there is no special need for a solvent. Indeed, numerous solid-state organic reactions have been found in which the solid state reaction occurs more efficiently and more selectively than its solution-phase counterpart. Other advantages include low-costs, simplicity in process and handling, as well as reduced pollution. Indeed, such processes should work for many reactions that are completely dry, paste-like, or generally where the reagent can act as the solvent. See Tanaka et al., Chem. Rev., 2000, 100:1025-1074, incorporated herein by reference.